The present invention relates generally to the field of orthopaedics, and more particularly, to a manufacturing method and molding die for an implant for use in joint arthroplasty.
The invention relates to joint prostheses. More particularly, the invention is directed to tibial components of knee joint prostheses that can be configured to be either rotatable or non-rotatable.
Joint replacement surgery is quite common and it enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints usually comprise metallic, ceramic and/or plastic components that are fixed to existing bone.
Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. A typical knee prosthesis includes a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing insert. The femoral component generally includes a pair of laterally spaced apart condylar portions, the distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing insert.
The tibial plateau is mounted within the tibia of a patient. Typically, the tibial bearing insert, which is usually, made of ultra high molecular weight polyethylene (UHMWPE), is mounted upon the superior surface of the tibial plateau. The geometry and structure of the tibial bearing insert varies depending upon the needs and joint condition of a patient. Some tibial bearing inserts are designed to be used with joint prostheses that are implanted during procedures that retain one or both of the cruciate ligaments. Others are implanted after removal of one or both cruciate ligaments, and are thus structured to compensate for the loss of these ligaments. Yet other tibial bearing inserts are used with prostheses that provide enhanced stabilization to the knee joint.
Recent total knee prostheses have been designed which allow for increased freedom of rotation between the femur and the tibia. To allow for this rotational motion, tibial bearing inserts have been designed which allow for rotation of the insert on the tibial tray or plateau. Typically the tibial bearing inserts have a central stem which rotationally engages centrally in the tibial stem of the tibial tray implant thereby providing for the rotational motion. Typically, there are no rotational constraints between the tibial tray implant and the tibial bearing insert. Frequently during total knee arthroplasty, the posterior cruciate ligaments are sacrificed and a substitute for the posterior cruciate ligaments is required. Orthopaedic implants for total knee arthroplasty have been developed which provide for the substitution of the posterior cruciate ligament. Examples of such implants include the PFC Sigma RP as described in U.S. Pat. No. 4,298,992 incorporated herein by reference and the LCS Complete total knee prosthesis, both of which are sold by DePuy Orthopaedics, Inc., Warsaw, Ind.
These total knee prostheses are designed with tibial components and femoral components which have in conjunction with their articulating surface, a spine and cam mechanism, which is used as a posterior cruciate substituting feature when the posterior cruciate of the knee is sacrificed.
Such total knee replacement prostheses, which include a spine and cam mechanism, typically contain tibial bearing components manufactured from suitable plastic, usually UHMWPE. One such construction use for a class of total knee replacement prosthesis which is known as constrained prosthesis often incorporate metal reinforcement rods in the construction of the plastic bearing component. The bearing insert is constructed so that the metal rod lies within the bearing and thus provides additional support for the central spine element of the bearing. Such components are typically manufactured by machining or molding the bearing component, drilling a central hole, and press fitting the reinforcing metal rod. An example of such a component is described in U.S. Pat. No. 5,007,933 to Sidebotham, et al. hereby incorporated in its entirety by reference.
In order to allow for desired kinematics of the knee during a full range of motion, the spine and cam mechanism on the tibial bearing insert may be placed in a suitable position, preferably anterior to the center line of the insert in the anterior/posterior direction. Designs of tibial inserts are available to help reconstruct knees where the stabilizing soft tissue compromises have been made or occurred due to various reasons. In such cases, the tibial bearing inserts are required to experience greater loads in the anterior/posterior and the medial/lateral directions. The constrained inserts may be reinforced with a metal rod as mentioned earlier to help distribute the loads experienced by the spine of the polyethylene tibial bearing.
Total knee joint prostheses have been designed with the spine and cam mechanisms on the tibial bearing insert placed in a position that the central axis of the distal stem portion of the insert that engages the tibial tray and the axis of the superior spine portion that engages the cam of the femoral component are not necessarily collinear.
Unfortunately, this design does not allow for a straight rod, commonly employed for reinforcement of tibial bearing inserts, to be used.
It should be appreciated that a first rod could be inserted inside the spine and a second rod could be inserted in the stem of the tibial tray portion of the bearing insert. However, the load on the first rod would be transferred through the polymer portion of the insert to the second rod. The polymer strength would then limit the load carrying capacity of this configuration. Such a configuration may not provide the required strength to sufficiently support and reinforce the spine.
A manufacturing method to produce a tibial bearing insert that may rotate about the tibial tray is thus required which has the strength required for the greater loads in the anterior/posterior and medial/lateral direction of the spine. The present invention is directed to providing a tibial bearing insert manufacturing method and tools to perform the same with sufficient strength at the spine to withstand the loads of the mobile knee prosthesis in the anterior/posterior and medial/lateral direction.
The present invention is directed to tools and methods for making an improved joint prosthesis for total knee replacement that includes a spine and cam mechanism and a distal rotating stem. The cam mechanism being on the femoral component and the spine and distal stem being on the bearing component. The mechanism is capable of withstanding the greater loads experienced in the anterior/posterior and medial/lateral direction caused by the substitution of the cam and spine for the cruciate ligaments, which may be sacrificed during total knee arthroplasty.
The spine on the tibial bearing insert made from a method and utilizing a mold according to the present invention is placed anterior to the centerline of the insert in the anterior posterior direction. Therefore, the distal stem portion of the insert that engages the tibial tray and the superior spine portion which engages the cam of the femoral component are not in the same plane. The tibial bearing insert manufacturing method of the present invention thus includes providing a reinforcing rod placed internal to the tibial bearing insert which includes an offset feature to accommodate such plane differences.
The rotating bearing of a knee prosthesis manufacturing method of the present invention thus includes the step of providing polymeric material with reinforcing a first component including a first portion on a first center line and a second portion on a second center line such that the first portion within the polymeric material distal stem may engage the tibial tray and the second portion within the polymeric material spine may be cooperating with the cam mechanism in the femoral component of the knee prosthesis.
According to one embodiment of the present invention, there is provided a method of manufacturing a polymeric bearing component for use in joint arthroplasty. The method includes the step of providing a non-linear reinforcing support of a durable material having a first end and a second end. The method further includes the step providing a molding die adapted for manufacturing the bearing component for use in total joint arthroplasty and having a first mold portion and a second mold portion. The first mold portion is adapted to provide a first surface of the bearing component for cooperation with the first joint component and the second mold portion is adapted to provide a second surface of the bearing component for cooperation with the second joint component. The method further includes the steps of positioning the support in a desired position within the molding die with one of the first end and the second end located in the first mold portion, adding moldable polymeric material into the molding die, substantially surrounding the support with the moldable material, heating and pressurizing the mold, permitting the moldable material to cool to form the bearing component, and removing the component from the molding die.
According to another embodiment of the present invention there is provided a molding die for use to mold a moldable material over a substantial portion of a reinforcing support having first and second ends. The molding die provides an article for use in joint arthroplasty. The molding die includes a first mold portion having a first forming surface and a second mold portion having a second forming surface and being mateable with the first mold portion. The first die portion and the second mold portion form a cavity there between when the first mold portion and the second mold portion are mated, the first forming surface and the second forming surface define at least a portion of the outer periphery of the article. The molding die also includes a positioner for spacing the reinforcing support within the cavity with at least a portion of the support spaced from at least one of the first forming surface and the second forming surface. The positioner is adapted to position the first end of the reinforcing support adjacent the first forming surface and adapted to position the second end of the reinforcing support adjacent the second forming surface.
The technical advantages of the present invention include the ability to allow for the desired kinematics of the knee during a full range of motion for patients in which the cruciate ligaments have either been severely damaged or have been removed or sacrificed. In such conditions the femoral and tibial components of the knee prosthesis need to be constrained with respect to each other by use of, for example, a spine and cam mechanism.
According to another aspect of the present invention, the reinforcing support member may be placed within a molding die and a tibial bearing insert may be molded incorporating the a non-linear support member.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims.